When weight is lost, the majority of it is breathed out as carbon dioxide.

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Prof. Andrew Brown and Ruben Meerman reported widespread misconception regarding how weight is lost, finding physicians, dietitians and personal trainers all equally guilty of not knowing. Most believed that fat is converted to energy or heat, "which violates the law of conservation of mass," they write.

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They found that when 10 kg of fat were oxidized, 8.4 kg were converted and excreted as carbon dioxide (CO2) via the lungs, and 1.6 kg became water (H20).

"None of this biochemistry is new," say the authors, "but for unknown reasons it seems nobody has thought of performing these calculations before. The quantities make perfect sense but we were surprised by the numbers that popped out."